Radiotherapy consists of the application of harmful radiation to lesions (such as tumours) within the body. The radiation interferes with cellular processes within the lesion and alleviates the condition. A principal concern within radiotherapy is to maximise the dose that is delivered to the lesion and minimise the effect of dose that is delivered to surrounding healthy tissue. This is done in a number of ways.
One is to deliver the dose in a number of “fractions”, separated over time such as on sequential days. Another is to deliver the radiation from multiple different directions, thus delivering only a partial dose to the surrounding tissue. Often, this is done by mounting the radiation source on a gantry that is rotatable around an axis and directed along a beam direction that co-incides with that axis, thus delivering radiation from (potentially) all directions within a plane transverse to the axis. Another method is to shape the cross-sectional profile of the beam using suitable collimators, such as to conform to the external cross-section of the lesion when viewed along the beam axis, or to another shape calculated to deliver a dose that will build up towards a desired three-dimensional dose distribution. Usually, some combination of all three will be employed.
In relation to the collimation of the beam, most tumours can be dealt with using a combination of block collimators and a so-called “multi-leaf” collimator. A block collimator is a solid block of radiopaque material such as tungsten, which usually has a straight front edge that spans the entire width of the device's aperture, and which can be advanced and/or withdrawn across the aperture in a direction transverse to the front edge. Thus, the block collimator has the effect of adjusting the width of the aperture as needed. A pair of such collimators arranged face-to-face can thus narrow the aperture from both opposing sides.
A multi-leaf collimator such as the one disclosed in our earlier application EP-A-0,314,214 comprises an array of long, narrow, deep leaves of radiopaque material that can each be extended into and out of the aperture. Arranged side-by-side, the tips of the leaves therefore define a chosen shape which can be varied at will by extending or retracting individual leaves.
Both the leaves and the block collimators usually have rounded tips in order to reduce the penumbra that they cast and thus improve the definition of the beam that is allowed through. This does therefore mean that opposing collimator elements cannot be extended so that they meet and close the field completely. Usually, an aperture will be collimated by a pair of opposed block collimators operating in one direction (say, the y direction) and a pair of opposed multi-leaf collimator (“MLC”) banks operating in the transverse direction (say, the x direction), both directions being transverse to that of the beam (the z direction). Our earlier application WO2008/141667 discloses a design for the block collimator leaves which can co-operate with the MLC leaves so as to minimise the necessary weight of the block collimator elements; the reader is specifically directed to that disclosure (which is incorporated herein by reference) for a fuller understanding of the present invention.
To produce very small radiation fields, the resolution of a standard MLC and block collimator is usually too coarse, and the rounded tips of both are unsuited to forming very narrow beams. Therefore, one of a micro-MLC or “stereotactic cones” are often used. A micro-MLC is akin to a standard MLC but much smaller. This is however very difficult to engineer, as the leaves are very thin and therefore less rigid. Stereotactic cones offer a range of preset shapes and sizes, such as 5 mm, 10 mm, 15 mm and 20 mm diameter circles, and are provided as “add-on” collimators that can be attached to the radiation head as and when needed. These are usually fitted to the exterior of the radiation head, in the path of the beam, so as to place them as close as possible to the patient for best accuracy. As an alternative, US2014/0048727 discloses stereotactic cones that are integrated into the beam generation system adjacent the primary collimator; this will be more convenient for the operators but at the expense of lesser accuracy in delivery.